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ASL has very different structure to spoken/written English, so not everybody who signs is going to comprehend English grammar as fluently/easily or the nuance of all the words that don't have a sign equivalent.
Additionally ASL communicated who is talking and the tone of their words, even when the speaker is off screen, which just can't be captured by captioning. Closed captioning has just caught on to using slightly different colors to indicate the speaker, so you know who's talking offscreen. I've only seen this in British panel shows so far but it's helpful.